Silicon solar cells require metal electrodes to be formed to each of the p-type and n-type semiconductor material of the solar cell to allow light-generated charge carriers to be extracted and flow in external electrical wires as electricity. Most silicon solar cells have one polarity of electrode on the top (illuminated) surface and the opposite polarity metal contact on the rear surface. In general, most silicon solar cells use different metals for the two cell surfaces because of the differing functions of the different polarity surfaces. For illuminated surfaces, shading losses, metal conductivity and contact resistance to the semiconductor are particularly important, whereas a higher metal coverage and lower conductivity can be tolerated on non-illuminated surfaces. In screen-printed silicon solar cells screen-printed silver is typically used to contact the illuminated n-type front surface and screen-printed aluminium is typically used to contact the p-type rear surface.
Metal electrodes can be formed to solar cells via various metal plating processes. Metal plating is the electrochemical deposition of metal from a solution of metal ions. Deposition can be achieved using an electroless process in which a reducing agent is added to the solution of metal ions to provide a source of electrons at the silicon surface for the metal reduction process. Alternatively, the source of electrons can be the light-induced current of a solar cell in light-induced plating (LIP) or provided by an applied electrical potential in an electroplating process. Electroless plating has been used to form nickel/copper metal electrodes to heavily-doped n-type and heavily-doped p-type grooves in silicon solar cells. Electroless plating has been used commercially by BP Solar in the manufacture of their laser buried grid silicon solar cells. More recently, LIP has been successfully used to metallise a range of silicon solar cells including laser-doped selective-emitter (LDSE) cells. In LIP, the n-type silicon is typically exposed at the base of grooves formed in a front-surface dielectric layer. The LIP process typically requires that the rear surface of the cell either acts as the anode or is physically contacted to an anode. The anode is then oxidised to maintain the source of metal ions for the plating process.
For cells that have a rear aluminium electrode, LIP is an attractive process because the process can be either contactless (as described by Lawrence Durkee in U.S. Pat. No. 4,144,139 “Method of Plating by Means of Light”), or if physical contact is used, then the cell contacting requirements are relatively straightforward because the electrical contact can be made anywhere on the rear aluminium surface (i.e., alignment is not critical). Once physical low-resistance contact has been made to the rear electrode of the cell, then an applied bias can be used to: (i) cathodically protect the aluminium electrode from oxidizing; and (ii) bias the solar cell to operate closer to its maximum (short circuit) current.
Since LIP uses the light-induced current generated by the solar cell to reduce the metal ions to form metal contacts, it can only be used to metallise n-type regions of solar cells. It cannot be used to metallise the p-type regions of cells fabricated on n-type wafers or the p-type regions of bifacial cells. In these cases, exposing the cell to light makes the exposed p-type silicon anodic and hence there is no source of electrons for metal reduction. If low-cost nickel/copper metal plating is to be used for these types of solar cells, then currently either electroless plating or electroplating must be used. Electroless plating has been demonstrated to be undesirable in silicon solar cell manufacturing because of the long plating times required and the expense of maintaining the plating baths and disposing of waste electrolyte. Although electroplating can use much simpler chemistry, because reducing agents are not required in the solution, physical low-resistance contact must be formed to the heavily-doped regions to be plated. This can require precise alignment, especially if the doped regions have been patterned very finely to reduce front surface shading losses. Furthermore, the regions have to be sufficiently conductive for uniform plating over the entire metallisation (i.e., grid) pattern. This typically necessitates a seed layer metal being formed in the regions to be metallised by some other process such as evaporation, sputtering or printing. This complicates the metallisation process.
A key step in the manufacture of silicon solar cells, which can directly result in higher efficiencies by way of increased open circuit voltages, is the passivation of the surfaces of silicon wafers. The passivation processes that currently dominate industrial silicon solar cell passivation include plasma-enhanced chemical vapour deposition (PECVD) for silicon nitride (SiNx) and aluminium oxide (AlOx), thermal oxidation for silicon dioxide (SiO2) and atomic layer deposition (ALD) of AlOx.
Anodic oxide passivation has been suggested as an alternative way for forming oxide layers which can be effectively patterned due to their porosity.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.